Conventionally, as disclosed in the Official Gazette of Japanese Laid-open Patent Application No. 138371/1981, in a solid state image sensor such as a CCD or the like, there has been considered a method whereby excessive carriers are extinguished using surface recombination for prevention of blooming in place of providing overflow drains in the light receiving surface.
This method has advantages such that sensitivity is high since the opening ratio of the light receiving surface is not sacrificed and horizontal resolution power is raised since the degree of integration can be improved, and the like.
FIGS. 1 to 3 are diagrams for explaining such a blooming preventing method due using surface recombination, in which FIG. 1 is a front view showing an ordinary frame transfer type CCD.
In the diagram, a light receiving part 1 consists of a plurality of vertical transfer registers having photosensitivity.
A storage part 2 consists of a plurality of vertical transfer registers which are shielded against the light.
A reference numeral 3 denotes a horizontal transfer register. The information in each vertical transfer register in the storage part 2 can be stored in this horizontal transfer register by simultaneously shifting it by one bit. Then, by allowing the register 3 to perform the horizontal transfer operation, a video signal can be derived from an output amplifier 4.
In general, the information formed in each vertical transfer register in the light receiving part 1 is transferred vertically to the storage part 2 within a vertical blanking interval based on the standard television system and is sequentially read out line by line by the horizontal transfer register 3 within the next vertical scan interval.
It is now assumed that the light receiving part 1, storage part 2 and horizontal transfer register 3 are each two-phase driven and respective transfer electrodes are indicated by P.sub.1, P.sub.2, P.sub.3, P.sub.4, P.sub.5, and P.sub.6, and respective transfer clocks are represented by (.phi..sub.p1, .phi..sub.p2), (.phi..sub.p3, .phi..sub.p4) and (.phi..sub.p5, .phi..sub.p6).
FIG. 2 is a diagram showing a potential profile below such transfer electrodes P.sub.1 to P.sub.6. For example, a low-potential portion and a high-potential portion (as seen by electrons) are formed by means of ion implantation or the like below each electrode which is provided over a P-type silicon substrate 6 and separated thereform by an insulation layer 5. For instance, when a voltage -V.sub.1 at a low level is applied to the electrodes P.sub.12, P.sub.4 and P.sub.6 and a voltage V.sub.2 at a high level is applied to the electrodes P.sub.1, P.sub.3 and P.sub.5, potentials as indicated by a solid line in the diagram are formed. On the other hand, when a low-level voltage V.sub.1 is applied to the electrodes P.sub.1, P.sub.3 and P.sub.5 and the high-level voltage V.sub.2 is applied to the electrodes P.sub.2, P.sub.4 and P.sub.6, potentials as indicated by a broken line in the diagram are formed.
Therefore, by applying alternating voltages having mutually opposite phases to the electrodes P.sub.1, P.sub.3, P.sub.5 and the electrodes P.sub.2, P.sub.4 and P.sub.6, the carrier is sequentially transferred in one direction (to the right in the diagram).
On one hand, an alternate-long-and-short dash line in FIG. 2 indicates potentials in the case where a large positive voltage V.sub.3 is applied to the electrodes. These potential wells are in the reversed state, so that excessive carriers over a predetermined amount will have been extinguished by recombining with the majority carriers.
FIG. 3 is a diagram showing the relation between the voltage applied to the electrodes and the shape of the internal potential as mentioned above with respect to the direction of thickness of the semiconductor substrate 6. It will be understood from the diagram that the potential wells are shallow for the electrode voltage V.sub.3 and the excessive carrier enters the second state in that it can recombine with the majority carrier at the interface with the insulation layer.
On the other hand, the accumulation state as the first state occurs at the electrode voltage -V.sub.1, so that the majority carrier can be easily collected around the interface and, for instance, this majority carrier can be supplied from a channel stopper area (not shown).
Therefore, for instance, by alternately applying the voltages -V.sub.1 and V.sub.3 to the electrode P.sub.1 while a barrier is formed and maintained by applying the voltage -V.sub.1 to the electrode P.sub.2, the minority carrier which is accumulated below the electrode P.sub.1 is limited to less than a predetermined amount.
However, such an arrangement as shown in FIG. 2 has the drawback that its efficiency in removal of the excessive charges is low. Namely, in transfer electrodes, transfer efficiency is generally improved by preventing the surface recombination of charges upon storage and transfer of charges. For this purpose, the potential well below the transfer electrode has to be sufficiently deep and the area of the bottom surface of the well has to be wide.
On the other hand, the opposite conditions are needed to efficiently recombine the charges.
That is, it is desirable that the area of the bottom surface of the potential well below the electrode be narrow to a certain extent so that the charges efficiently overflow and are recombined.
Consequently, in the conventional technology, there occurs a problem that an increase in transfer efficiency causes the efficiency in recombination to be deteriorated and, contrarily, an increase in recombination efficiency results in a decrease in transfer efficiency.
In addition, in such a CCD using charge recombination, there is a problem that the clock for recombination is mixed with the output signal and becomes noise of a fixed pattern. Therefore, a onventional apparatus is arranged such that the recombination clock is supplied only during the horizontal blanking interval.
In such a CCD of the conventional type, the frame transfer is started within the vertical blanking interval; therefore, the generation of the recombination clock is stopped during the interval of, e.g., about 1/2 H (horizontal interval) immediately before that interval. Thus, a charge transfer is started in the state in which a great amount of charges has been stored which may cause a smear phenomenon or blooming phenomenon. Therefore, a method whereby the device is driven even during the vertical blanking interval is also considered. However, this method is undesirable because of the problem of a large electric power consumption or the like.